Direct current electric energy is generated during new energy grid-connected power generation, the direct current electric energy needs to be converted into alternating current electric energy by using an inverter, and then power is supplied to a main electrical network and a load. An island state of the inverter means that when a grid-connected switch trips, the inverter is still connected to the main electrical network because the inverter cannot detect a power-off state of the electrical network, and supplies power to the load as a separate power supply.
There are the following major potential risks caused by the island state for devices and personnel: When corrective maintenance personnel stops power supply of the main electrical network, and performs corrective maintenance on electric power lines and circuit devices, when a new energy inverter still continues to supply power to the load, injuries and deaths of the corrective maintenance personnel are caused. In addition, if the grid connected inverter continues to supply power, after the main electrical network resumes power supply, a relatively great difference may exist between a voltage of the electrical network and an output voltage of the inverter in terms of a phase. Consequently, the device is damaged because a large impulse current is generated within a short moment.
Therefore, the island state needs to be detected by using a method such as communication or a peripheral, passive detection, or active disturbance according to an island detection policy, and the inverter needs to be enabled to disconnect from an island power generation system. However, the island detection policy usually depends on an over/undervoltage or over/underfrequency protection mechanism. When an over/undervoltage or over/underfrequency protection time of an inverter is longer than an island protection time stipulated in a grid connection standard, island detection fails.
Therefore, when a main electrical network is powered off, how to detect an island state of an inverter to disconnect the inverter from the electrical network becomes a problem that urgently needs to be resolved.